PROJECT SUMMARY The nuclear poly(A) binding protein 1 (PABPN1) is a ubiquitously expressed protein that plays critical roles at multiple steps in post-transcriptional regulation of gene expression. Short expansions of the polyalanine tract in the N-terminus of PABPN1 lead to Oculopharyngeal Muscular Dystrophy (OPMD). Patients who suffer from OPMD have progressive weakening of specific muscles most notably those of the pharynx. Defects in pharyngeal muscle function can cause choking and regurgitation leading to pneumonia or sudden death. There is no current treatment for OPMD. Much is still unknown regarding the mechanism by which ubiquitous expression of this alanine-expanded PABPN1 leads to muscle-specific pathology. We have discovered that the steady-state levels of both PABPN1 mRNA and protein are drastically lower in mouse and human skeletal muscle, particularly those impacted in OPMD, compared to other tissues. The low levels of PABPN1 in skeletal muscle could predispose this tissue to the deleterious effects of alanine-expanded PABPN1. The low level of Pabpn1 transcript present in muscle indicates tissue-specific regulatory mechanisms at either the transcriptional or post-transcriptional level or possibly both. We find that Pabpn1 expression in different tissues is in part regulated by a post-transcriptional mechanism that modulates transcript stability but further studies are needed to fully elucidate how PABPN1 expression is controlled. The goal of this proposal is to test our working hypothesis that low levels of PABPN1 in skeletal muscle predispose this tissue to the deleterious effects of alanine-expanded PABPN1. To define the mechanisms that lead to the low levels of expression of PABPN1 in muscle, we will map the cis-elements responsible for modulating the stability of the Pabpn1 transcript in skeletal muscle (Aim 1) and identify the cellular factors (RNA binding proteins and miRNAs) that bind to the PABPN1 transcript to modulate transcript stability in a tissue-specific manner (Aim 2). In addition, we will determine whether transcriptional mechanisms contribute to the low levels of PABPN1 transcript in skeletal muscle (Aim 3). Of particular importance to OPMD, we will exploit two novel mouse models (both a PABPN1 Knockout mouse and an alanine-expanded PABPN1 Knockin mouse) that we have create to directly test whether a decrease in the level of PABPN1 exacerbates the pathology induced by alanine-expanded PABPN1 (Aim 4). The long-term goal of our studies is to understand why ubiquitous expression of mutant PABPN1 leads to a muscle-specific disease. These studies are significant as they will identify pathways that could be manipulated to improve the quality of life for patients that suffer from OPMD. Furthermore, these studies could lay the groundwork for understanding tissue-specific pathology in other diseases.